Effect of Biodegradation on CO2 Minimum Miscibility Pressure for the Frio Reservoir Fluids

Abstract

AbstractThe great Frio Salt-Dome oil fields dot the Texas coast south and southeast of Houston. Over 2.4 billion barrels of oil have been produced since these fields were discovered in the 1920s. Currently, oil is produced at water cuts higher than 95% from most of these fields. With deteriorating water flood economics, over 1.7 billion barrels of remaining oil are left to be targeted by CO2 flood. CO2 floods were successfully developed in a few Frio Fields. The current (on 1/1/2020) combined cumulative oil production from the Oyster Bayou and the West Hastings CO2 floods is over 30 MMSTB. Even with the high volume of remaining oil and the great success of the CO2 floods, some of the Frio fluids are a challenge for CO2 flooding due to biodegradation of the oils. Local displacement efficiency depends on minimum miscibility pressure (MMP). Displacement efficiency is over 90% when operating pressure is above the MMP and drops significantly when operating pressure is lower than the MMP. In the Frio fields, however, the measured MMPs from the existing slimtube experiments were significantly deviated from what is estimated from most of the CO2 correlations. Therefore, these correlations can’t be used in estimating MMP during the screening phase of CO2 flood in the Frio fields. In this paper, the characteristics of different Frio fluids is shown with gas chromatograms and compositional analysis. The cause of biodegradation and its effect on CO2 MMPs in these Frio fields is discussed. A new parameter with the ratio of xC7+xC2−4 is identified to be an indicator of the level of biodegradation for the Frio oils. This parameter along with molecular weight (MW) of C5+ or C7+ and reservoir temperature is recommended as the new parameters for generating an improved CO2 MMP correlation for a general oil, while a simple CO2 MMP correlation is also presented for the Frio oil fields. Furthermore, guidance on screening fields from reservoir fluid aspects in the Frio Zone for CO2 flood is provided.